/**
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.hadoop.hbase.monitoring;

import java.io.PrintWriter;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.LinkedList;
import java.util.List;

import org.apache.yetus.audience.InterfaceAudience;

import org.apache.hbase.thirdparty.com.google.common.base.Charsets;
import org.apache.hbase.thirdparty.com.google.common.base.Preconditions;
import org.apache.hbase.thirdparty.com.google.common.collect.Lists;

/**
 * A size-bounded repository of alerts, which are kept
 * in a linked list. Alerts can be added, and they will
 * automatically be removed one by one when the specified heap
 * usage is exhausted.
 */
@InterfaceAudience.Private
public class MemoryBoundedLogMessageBuffer {
    private final long maxSizeBytes;
    private long usage = 0;
    private LinkedList<LogMessage> messages;

    public MemoryBoundedLogMessageBuffer(long maxSizeBytes) {
        Preconditions.checkArgument(maxSizeBytes > 0);
        this.maxSizeBytes = maxSizeBytes;
        this.messages = Lists.newLinkedList();
    }

    /**
     * Append the given message to this buffer, automatically evicting
     * older messages until the desired memory limit is achieved.
     */
    public synchronized void add(String messageText) {
        LogMessage message = new LogMessage(messageText, System.currentTimeMillis());

        usage += message.estimateHeapUsage();
        messages.add(message);
        while(usage > maxSizeBytes) {
            LogMessage removed = messages.remove();
            usage -= removed.estimateHeapUsage();
            assert usage >= 0;
        }
    }

    /**
     * Dump the contents of the buffer to the given stream.
     */
    public synchronized void dumpTo(PrintWriter out) {
        SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss");

        for(LogMessage msg : messages) {
            out.write(df.format(new Date(msg.timestamp)));
            out.write(" ");
            out.println(new String(msg.message, Charsets.UTF_8));
        }
    }

    synchronized List<LogMessage> getMessages() {
        // defensive copy
        return Lists.newArrayList(messages);
    }

    /**
     * Estimate the number of bytes this buffer is currently
     * using.
     */
    synchronized long estimateHeapUsage() {
        return usage;
    }

    private static class LogMessage {
        /** the error text, encoded in bytes to save memory */
        public final byte[] message;
        public final long timestamp;

        /**
         * Completely non-scientific estimate of how much one of these
         * objects takes, along with the LinkedList overhead. This doesn't
         * need to be exact, since we don't expect a ton of these alerts.
         */
        private static final long BASE_USAGE = 100;

        public LogMessage(String message, long timestamp) {
            this.message = message.getBytes(Charsets.UTF_8);
            this.timestamp = timestamp;
        }

        public long estimateHeapUsage() {
            return message.length + BASE_USAGE;
        }
    }

}
